Issue #90: Port setup.py to Python 3.9

[vstinner]

Avoid "U" mode in open(). This flag was ignored since Python 3.0 and
now raises an error in Python 3.9:
https://bugs.python.org/issue37330

On Python 2, setup.py no longer replaces DOS (\r\n) and Mac (\r) with
Unix newline (\n). But it is not an issue in practice, since
README.rst uses Unix newline format (\n).

Fix also a ResourceWarning: use "with" context manager to ensure that
the file is closed. The warning was displayed when using
"python3 -Wd setup.py (...)".

[vstinner]

Example without the fix on Python 3.9.0a4 on Fedora 31:

$ python3.9 setup.py build
Traceback (most recent call last):
  File "/home/vstinner/parameterized/setup.py", line 11, in <module>
    long_description = open("README.rst", "U").read()
ValueError: invalid mode: 'U'

It works as expected (no error) with the fix:

$ python3.9 setup.py build
(...)
copying parameterized/__init__.py -> build/lib/parameterized
copying parameterized/parameterized.py -> build/lib/parameterized
copying parameterized/test.py -> build/lib/parameterized

[vstinner]

@wolever: Would you mind to review this change?

[wolever]

Thanks! Fix will be out on 0.7.3 (pushing shortly)